University of Utah professor Jason Taylor said he was standing at his gate waiting to board his plane.

"The ball of fire lasted for several seconds," he said.

"It freaked a lot of people out.

"You don't often see fires on planes."

The plane was heading to Cincinnati, Ohio and scheduled to depart at 6:34am on Friday morning.

An airport spokesperson said: "The auxiliary power unit caught fire but when they turned the engine back off the fire extinguished itself and the plane was brought back to the gate and all passengers were de-boarded.